Most aircraft are equipped with altitude transmitting transponders to return (squawk) an aircraft's assigned squawk code and altitude when targeted by a radar transmission from ground control radar. To provide altitude information to the transponder, the cockpit altimeter may contain a digital interface to encode the position of the analog pointer or needle as a digital signal. Using that signal, the transponder, triggered by air traffic control radar, transmits a squawk signal according to the international ICAA code. While the typical system utilizes the dashboard altimeter to provide the input to the transponder, digitizing the pointer's position adds to the size and complexity of the cockpit altimeter. One technique uses an internal n channel optical encoder to resolve pointer position into 2" possible positions.
A specific ICAA altitude code is squawked when aircraft altitude is within a certain range or band. For instance, 10,000 feet would be squawked if the altitude is between 10,000 and 10,200 feet. At lower altitudes, the acceptable range (200 feet in the previous example) understandably decreases to maintain an acceptable percentage error. In the air traffic control system, air traffic control (ATC) receives the squawked signal as an altitude indication next to the echo or "blip" on a radar screen viewed by a controller tracking the aircraft. Pilots routinely report aircraft altitude, especially when changing altitude and flight levels and identifying the aircraft when entering an air traffic control center. If the two--the squawked and reported altitudes--do not agree, air traffic control notifies the pilot of the inconsistency. The squawked altitude is presumed to be wrong.